1. Field of the Invention
This invention relates to a process for effecting base-catalyzed reactions, such as aldol condensations and cyclizations, and the dehydrogenation, aromatization and isomerization of olefins using a basic zeolite as the catalyst.
2. Discussion of the Prior Art
The aldol condensation is an old and well known base-catalyzed reaction extensively described in the chemical literature and widely used to prepare a variety of compounds having industrial utility. The condensation is typified by the reaction of acetaldehyde with itself to form beta-hydroxybutyraldehyde when heated in a solution containing a base. Beta-hydroxybutyraldehyde, on further heating and/or when treated with acid, forms crotonaldehyde, due to the propensity of beta-hydroxy carbonyl compounds to eliminate water. The first step in the condensation is the formation of an enolate anion by removal of a hydrogen alpha to the carbonyl group in an acetaldehyde molecule: ##STR1## The second resonance form of the enolate anion adds to the carbonyl group of another molecule of acetaldehyde: ##STR2## Beta-hydroxybutyraldehyde is formed when the adduct combines with a proton. It is important to note that both enolate formation and addition are equilibrium reactions.
Other aldehydes and ketones having a hydrogen alpha to the carbonyl group and capable of forming an enolate anion react in a similar fashion. For example, the aldol condensation of ketones is typified by the reaction of acetone with itself to form diacetone alcohol, and then, on dehydration, mesityl oxide.
It is not necessary that an aldehyde or ketone react with itself or that the carbonyl compound to which the anion adds also contain an alpha hydrogen atom. For example, acetone reacts readily with formaldehyde, which lacks an alpha hydrogen. Thus, it is possible to have mixed aldol condensations:
(a) aldehyde to another aldehyde; PA0 (b) ketone to another ketone; PA0 (c) aldehyde to ketone; PA0 (d) ketone to aldehyde.
If both carbonyl reactants have alpha hydrogens, it is apparent that two different enolate anions can be formed and a mixture of products may result. The amount of each product formed will depend on the equilibrium constants and rates of reaction for the anion-forming and addition steps.
It is also possible for a compound having two carbonyl groups to undergo an internal aldol condensation and cyclize. For example, when a solution of gamma-diketone containing a base is heated, a cyclopenten-1-one is produced.
Aldol condensations are generally effected in an aqueous and/or alcoholic solvent in the presence of a base, such as sodium hydroxide. The overall course of the reaction and yield of product obtained depends on the equilibrium constants and rates of reaction for the intermediate steps involved. The product is generally isolated by a series of steps involving acidification of the reaction mixture, extraction with a water-immiscible non-polar solvent, separation of the aqueous and organic layers, drying of the organic layer, and distillation to recover the product.
Other base-catalyzed reactions are described in the book titled "Base-Catalyzed Reactions of Hydrocarbons and Related Compounds", authored by Herman Pines and Wayne M. Stalick, Academic Press New York, 1977. For example, the base-catalyzed dehydrogenation and aromatization of dienes is discussed at pages 498 and 499; alkali metal amides and alkali metal naphthalenes are disclosed as catalysts for the conversion of cyclohexadienes to benzene. The isomerization of olefins, particularly 1-enes to 2-enes, is discussed at pages 26 and 27; metal amides, metal hydrides, organoalkali metal compounds, alkali metals supported on inert carrier materials, metal hydroxides and metal oxides are stated as having shown activity as catalysts for the isomerization of olefins.